1. Field of the Invention
The present invention relates to a motor having permanent magnets built in a rotor thereof.
2. Description of the Prior Art
It is known that in a magnetic motor a one-layer permanent magnets in a rotor made of a high permeability material such as iron. For example, in a prior art surface magnet motor, permanent magnets are attached to a surface of a rotor.
Recently, environmental issues have attracted intensive attention. In order to save energy, a motor with built-in permanent magnets, that is, with permanent magnets embedded inside a rotor, has been used to replace the surface magnet motor.
FIG. 1 shows an example of a prior art motor with built-in permanent magnets. The motor includes a rotor 3' and a stator 5. In the motor, each permanent magnet 17 has an arch-form and projects towards the center of the rotor 3' and is embedded inside a rotor core 3a' made of an iron core of a high magnetic permeability material or of silicon steel sheets. The motor shown in FIG. 1 has four poles, and four permanent magnets 17 are arranged along a circumferential direction of the rotor to have N and S poles which are alternately arranged. The stator 2 has teeth 6.
In the above-mentioned motor, there is brought about a difference between an inductance Ld in a d-axis direction (refer to FIG. 1) connecting the center of each permanent magnet with the center of the rotor, and an inductance in a q-axis direction (refer to FIG. 1) rotated by 90.degree. from the d-axis direction in terms of an electrical angle. Therefore, a reluctance torque is produced in addition to a magnet torque of the permanent magnets 17. A total torque T is expressed in Equation (1): EQU T=Pn*{.PSI.a*Iq+1/2(Ld-Lq)*Id*Iq}, (1)
wherein Pn denotes a number of pole pairs, .PSI.a denotes a magnetic flux in d-axis, Ld denotes an inductance in d-axis, Lq denotes an inductance in q-axis, Iq denotes a current in q-axis and Id denotes a current in d-axis. Equation (1) represents a voltage equation after the dq conversion. Magnet torque and reluctance torque are expressed in the first term and in the second term in a term expressed in parentheses { and } in Equation (1).
In the prior art surface magnet motor, since a magnetic permeability of the permanent magnet is approximately equal to that of air, the inductances Ld and Lq have nearly the same value, and therefore no reluctance torque is generated.
In contrast, in the prior art motor with built-in permanent magnets, the d-axis direction corresponds to a direction in which a magnetic flux is generated by the permanent magnets 17, and as shown in FIG. 1, a flow 21 of magnetic flux in the d-axis direction penetrates twice the permanent magnet having approximately the same magnetic permeability as air, thereby the d-axis inductance Ld is considerably reduced because of an increase in magnetic resistance. On the other hand, a flow 22 of magnetic flux in the q-axis direction is directed to a side face of the permanent magnet 17, passing the side face of the magnet as indicated in FIG. 1. As a result, the magnetic resistance is reduced and the q-axis inductance Lq is increased. The d-axis inductance Ld becomes consequently different from the d-axis inductance Lq. If a Id in d-axis current is supplied, the reluctance torque is generated.
FIG. 2 is a magnetic flux vector diagram illustrating this relationship. The magnet torque is generated by multiplying a magnetic flux .PSI.a with a current Iq in a direction perpendicularly electrically to the magnetic flux. The magnetic flux .PSI.a is a d-axis component of the total magnetic flux .PSI.0. Similarly, the reluctance torque is generated by multiplying magnetic fluxes Ld*Id, Lq*Iq with currents Iq, Id flowing perpendicularly to the magnetic flux, respectively. A sum of these two torques becomes the total torque T.
The total torque T depends on a phase .beta. of an input current I0, where Iq=I0*cos .beta. and Id=I0*sin .beta.. FIG. 3 shows a relationship of the magnet torque, reluctance torque and total torque when the current phase .beta. is changed while the current value is kept at I0. The magnet torque is maximum when the current phase is 0.degree., and it becomes smaller as the phase .beta. is increased, and it becomes zero when the phase is 90.degree.. On the contrary, the reluctance torque has a maximum value when the current phase is 45.degree.. Therefore, the total torque T becomes maximum in a range of 0-45.degree. of the current phase. Marks o indicate values obtained in an experiment, and the values agree well with values calculated according to Equation (1). That is, with the same current, a larger torque is obtained in the motor having permanent magnets embedded in the rotor, thereby to utilize the reluctance torque, than in the surface magnet motor.
Next, a problem of the prior art motor which has permanent magnets embedded in the rotor is explained. The reluctance torque is utilized to some extent in the motor. However, as indicated in FIG. 1, a flow 22 of the magnetic flux in the q-axis direction is obstructed by an end 17a of the permanent magnet 17 and cannot enter into the rotor. Most of the flow barely touches an outer peripheral part 18 of the permanent magnet 17. Thus, an amount of the magnetic flux is small, and the inductance Lq in q-axis cannot be increased.
As mentioned before, the larger the difference is between the inductances Lq and Ld (Ld is very small), the more the reluctance torque is generated by the same current. However, the q-axis inductance Lq cannot be increased so much in the prior art motor, and therefore the difference of the inductances Lq and Ld cannot be made large.